1. Field of the Invention
The present invention relates to a process for preparing layered thin films, more particularly, it relates to a process for depositing a plurality of thin films including a bottom superconductor layer on a substrate successively to produce a layered superconducting structure.
2. Description of the Related Art
Oxide superconductors are expected to be used in a variety of applications due to their higher critical temperatures than conventional metal superconductors. In fact, Y--Ba--Cu--O oxide superconductor possess the critical temperature above 80K and Bi--Sr--Ca--Cu--O and Tl--Ba--Ca--Cu--O oxide superconductors possess that of above 100K.
When the oxide superconductors are used in superconducting devices, it is indispensable to prepare a thin film of oxide superconductor and to laminate a plurality thin films including a bottom superconductor layer successively on a substrate. For example, in a superconducting junction so-called tunnel type Josephson Junction realized with oxide superconductor, it is requested to deposit a bottom superconductor layer, an intermediate layer of non-superconductor and a top superconductor layer on a substrate successively in this order.
Josephson element is a two-terminals element, so that a logical circuit consisting of Josephson elements alone becomes complicated. In order to overcome this demerit of complexity, a variety of ideas of three-terminals elements are proposed. In the superconductor transistors consisting of superconductor and semiconductor which is a typical three-terminals element, it is also required to combine a thin film of semiconductor with a thin film of oxide superconductor and hence successive deposition of thin films each consisting of different material is required.
Successive deposition of a thin film of ordinary conductor such as metal onto a thin film of oxide superconductor is required also in the other type superconducting element consisting of superconductor and ordinary conductor.
In these superconducting elements, a superconducting current passes through a thin film of non-superconductor sandwiched between two adjacent layers of superconductors positioned close to each other. A distance between the two adjacent superconductors is determined by the coherence length of superconductor. Since the coherence length of oxide superconductors is very short, the distance between two adjacent superconductors must be several nanometer.
In addition to this, from a point of view as performance of the superconducting devices, all thin films in the superconducting device must have high crystallinity, in other words, these thin films are made preferably of a single crystal or polycrystal having crystal orientation which is similar to single crystal. When the superconducting device has thin film(s) made of polycrystal whose crystal orientation is not well-ordered or has amorphous thin film(s), high-performance of the superconducting device can not be expected and hence function thereof become unstable.
When a plurality of thin film of oxide superconductors are deposited successively on a common substrate, it is usual practice to subject a surface of a bottom superconductor layer to cleaning operation before a top superconductor layer is deposited, otherwise electrical continuity between the bottom superconductor layer and the top superconductor layer is spoiled due to contaminants adsorbed on a surface of the bottom superconductor layer or undesirable oxides produced on the surface. Discontinuity of two layers result in formation of an undesirable junction between two layers. Superconducting devices or integrated superconducting circuits having such undersirable junction do not show desired performance and sometimes do not work.
In particular, the surface condition of the bottom superconductor layer should be considered carefully because the coherence length of oxide superconductors is very short and hence the superconductivity is easily lost. Therefore, the surface of bottom superconductor layer must be clean and also must have well-ordered crystallinity or superconducting property.
In the field of semiconductor industries, surfaces are cleaned with ultra-pure water, by chemical washing, dry or wet etching or the like. In the case of oxide superconductors, however, these clearing techniques can not be used due to high reactivity of oxide superconductors. If the surface of thin film of oxide superconductor is treated by these known techniques, undesirable reaction occur on the surface, resulting in that cleanness of the surface become worse and crystallinity and superconducting property are lost.
Under certain film forming materials and conditions, constituent elements of the top superconductor layer diffuse or migrate into the bottom superconductor layer, resulting in that an interface between these layers becomes unclear and thin films of oxide superconductor are deteriorated.
An object of the present invention is to solve the problems and to provide a process for preparing layered thin films comprising a bottom superconductor layer and the other thin film layers deposited on the bottom superconductor layer and having improved crystallinity and continuity especially at superconducting interfaces.